The present invention relates to control systems of industrial processes, and particularly to tuning a process control loop.
In process industries, such as pulp and paper, refining, petrochemical and chemical industries, the material flows in the process are controlled by a variety of control valves installed in a piping system of a plant. The material flow may include any flowing material, such as fluids, slurries, liquid, gases and steam. In the simplest configuration the control valve may be a manually adjustable mechanical valve. However, usually an automated control of the valve is used, and therefore the control valve is often provided with a valve controller and an actuator. The valve controller and the actuator adjust the position of the control valve (e.g. the opening of the valve) according to a control input (e.g. pneumatic or electric control input) from the process control system. The valve controller may also be called a valve positioner.
FIG. 1 shows a functional block diagram of a control valve. A valve controller (i.e. a positioner) 10 controls the valve travel/position (h) with the help of a torque generated by an actuator 11. Position information (h) is fed back from the actuator 11 or a control valve 12 to an adder in the input of the valve controller 10. Operation of the valve controller is mainly based on an error (e) between the input signal (the control signal from the process controller) and the position (h). The valve controller 10 operates so as to minimize the error by a control algorithm, such as a state or PID algorithm. The control algorithm is tailored for each control valve and can be tuned when installed or during operation, if necessary. The tuning may include change of the gain parameters, for example. In the valve controller 10 it is also possible to use one or more additional feedbacks, such as a velocity feedback and pressure feedback of an actuator cylinder, to achieve a more balanced and accurate valve position control action. For example, the gain parameters of the control algorithms may be changed according to the velocity of the valve. A valve controller, an actuator and a valve are normally integrated to form a field device.
FIG. 2 illustrates a typical model of a process control loop which controls one control valve 22 and thereby one material flow in the process. An industrial process may include hundreds of control valves and respective process control loops. The process control loop includes a process controller 21 with a control algorithm which provides a control signal Pc1 to the control valve 22 according to a setpoint value (from the control room computer, for example) and a measured process variable. The control algorithm may be any algorithm used in control systems, such as PID. The control signal Pc1 entered into the control valve 22 controls the valve position, the valve travel and thereby the material flow in the process. A desired process variable is measured by a measurement transmitter 24 and compared to the setpoint value of the same process variable so as to provide an error signal e2 which is entered into the process controller 10. The process controller""s function is to minimize the control error. The process control error is typically caused by setpoint changes and process disturbances. Control errors can not be completely removed because of time delays in the processes and the field devices. Further, accuracy is limited in measurements and control. When installed, the process controller 21 is tuned to operate as well as possible in different process load situations and at different operation points. The tuning is normally carried out by causing disturbance in the process and measuring how well the process controller handles them.
Traditionally the valve controller, or any field device controller, has been tuned alone independently from the process controller. However, this traditional tuning is not the optimal tuning method, since there actually is interaction between the two controllers. The inventors have observed that a valve controller which is optimally tuned from the valve point of view may result in a degradation of the performance of the process controller. On the other hand, tuning the process control may degrade the performance of an xe2x80x9coptimallyxe2x80x9d tuned valve controller.
An object of the invention is to provide a tuning method and a tuning system which provide a more optimal performance of a process control loop.
An aspect of the invention is a method for tuning a process control loop controlling material flow within an industrial process, said process control loop comprising a process controller, a field device controller, a field device, a first feedback from the field device to the field device controller, the industrial process and means for measuring a process variable in the process and for providing a second feedback from the process to the process controller, said method comprising steps of
tuning the process controller,
fine-tuning the field device controller and the process controller by finding for the two controllers control parameters, whereby the interaction of the controllers provides a desired process variability.
An another aspect of the invention is a control system comprising
a process control loop controlling material flow within an industrial process, said process control loop comprising a process controller, a valve controller, a field device in the process, a feedback from the field device to the field device controller, and a process variable feedback from the process to the process controller,
a tuning system for the process control loop, said tuning system being arranged to fine tune the field device controller and the process controller by finding for the two controllers control parameters, whereby the interaction of the controllers provides a desired process variability.
In the present invention, both the process controller and the field device controller, such as a valve controller, are fine-tuned so that their interaction is taken into account in order to find control parameters which provide an optimal control loop performance are found. This results in a significant improvement in the control loop performance, i.e. in a lower process variability, as compared with the traditional tuning method which does not take into account the interaction between the two controllers.
In a preferred embodiment of the invention a rough tuning of the process controller is first made during normal process operation using a combined field device and process model, i.e. the field device, e.g. a valve, is considered as part of the process. Thereafter, a fine-tuning of the process controller and the field device (e.g. valve) controller is done in one or more tuning cycles so that the field device controller is tuned to work optimally with the process controller and vice versa, by finding the control parameters which provide a desired control loop performance.
In an embodiment of the invention the fine-tuning is carried out during a process shut-down. In this embodiment a real field device, such as a valve, is used to control a simulated process. In other words, the process controller controls a real valve and a measured valve position is fed into the simulated process, for example.
In yet another embodiment of the invention, the fine-tuning is carried out using a simulated process and a simulated field device (e.g. a valve) which is controlled by the process controller. This embodiment allows an offline tuning of the process controller even during normal process operation, without disturbing the process. Upon finding the optimal field device control parameters (such as valve control parameters), the parameters are fed into the field device controller controlling the real field device, e.g. a valve.